Throughout the human body, 500-600 lymph nodes are situated. These secondary lymphoid organs collect antigens from peripheral tissues via the afferent lymphatics and provide T and B lymphocytes with the optimal environment for cellular activation and proliferation. In this chapter, we will highlight the interactions between hematopoietic cells and stromal cells that are essential for the proper formation and organization of the lymph node. In addition, the distinct cellular architecture that is characteristic for secondary lymphoid organs as well as its vascular system will be discussed in the context of lymph node function.